Generally, a high voltage DC circuit breaker refers to a switching device that can interrupt an electric current flowing through a high voltage (i.e. 15 kV or higher) transmission line, such as a high voltage direct current (HVDC) transmission system. The high voltage DC circuit breaker functions to interrupt a fault current when a certain fault occurs in a DC transmission line. This also can be applied to a medium voltage DC distribution system that distributes electric power of medium-level voltages ranging from 1 to 50 kV.
As to a high voltage DC circuit breaker, when a fault current occurs in a system, a main switch is opened to disconnect a faulty circuit, thereby interrupting the flow of a fault current from the faulty circuit. However, since there is no zero current point on a DC transmission line, when a main switch is opened, an arc generated between terminals of the main switch is not extinguished. Therefore, a fault current flows through the arc. That is, the fault current fails to be interrupted.
FIG. 1 shows the technology of a high voltage DC circuit breaker disclosed by Japanese Patent Application Publication No. 1984-068128. According to this technology, in order to interrupt a fault current Ic by extinguishing an arc generated when a main switch CB is switched off, the high voltage DC circuit breaker superposes a resonance current of an LC circuit on a DC current IDC flowing through the main switch CB (i.e. Idc=IDC+IP), thereby creating a zero current point in the main switch CB. That is, when the main switch CB is closed, the resonance current Ip is supplied to be superposed on the DC current IDC. Then, the resonance current Ip becomes an oscillating current due to the LC resonance, and is amplified while passing through the main switch CB. Thus, when the magnitude of a negative resonance current −Ip increases to be larger than that of the DC current IDC, the fault current Ic becomes zero, and the arc in the main switch is extinguished. However, this conventional technology has the following problems: it needs to have a circuit rating two times larger than a rated current because a resonance current Ip larger than the DC current IDC needs to be superposed on the DC current IDC; and interruption speed is slow because multiple resonance cycles are required to generate a large resonance current Ip. In addition, the conventional DC circuit breaker has a problem that it cannot interrupt fault currents flowing in both forward and backward directions.
In order to solve these problems, a vacuum interrupter (VI) has been developed to prevent an arc from being generated when a main switch CB is switched off. However, it is difficult to apply an existing vacuum interrupter to a high voltage DC circuit breaker due to a low rated voltage thereof.